1. Field of the Invention
This invention relates to electron trapping optical materials and a process for making and using such material.
2. Description of the Prior Art
In order to define the family of materials involved, it is useful to review the history, particularly since sometimes confusion exists over terminology. It is important to begin with the term luminescence, the ability of certain solids to emit light under different conditions.
Luminescence is a long known phenomenon of nature reaching back very far in history. Recorded observations reach back to the last century. Seeback and Becquerel observed momentary visible afterglow in certain materials. In 1889, Klatt and Lenard also observed some effects with infrared. During this time period, words like "phosphor" and "luminescence" appeared. In 1904, Dahms distinquished between "stimulation" and "quenching"; meaning inducing or stopping afterglow. Much of the later work is associated with Lenard, who received the Nobel Prize in 1905 in physics for cathode ray emission. He studied different phosphors until at least 1918. Later work can be found by Urback in 1926 through 1934. These early scientists basically observed very small luminescent effects.
In 1941, a program was instituted by the National Defense Committee for development of light emitting phosphors. The work started at the University of Rochester, and other laboratories become involved; however, the projects ended with World War II. The following technical papers were published on this work between 1946 and 1949:
B. O'Brien "Development of Infrared Phosphors", J. Opt. Soc. of Am., vol. 36, July 1946, p. 369;
F. Urbach, et al., "On Infrared Sensitive Phosphors", J. Opt. Soc. of Am., vol. 36, July 1946, p. 372;
G. Fonda, "Preparation and Characteristics of Zinc Sulfide Phosphors Sensitive to Infra-Red", J. Opt. Soc. of Am., vol. 36, July 1946, p. 382;
A. L. Smith, "The Preparation of Strontium Selenide and its properties as a Base Material for Phosphors Stimulated by Infra-Red", Journal of the Am. Chem. Soc., vol. 69, 1947, p. 1725; and
"Preparation and Characteristics of Solid Luminescent Materials", Editors: G. R. Fonda and F. Seitz. John Wiley & Sons, Inc., New York, 1948.
These papers provide an early story on the materials studied, As decades went by, the effects were forgotten by most physicists. Only work in the field of cathodoluminescence for screens of cathode ray tubes and fluorescent lamps continued with any focus.
Thus, the field of luminescence is broad and refers to the ability of certain substances or materials to emit light when driven by an external energy source. When the driving energy source is light, the proper term is photoluminescence.
The most interesting class of materials are those which upon excitation by illumination can store electrons in "traps" for varying lengths of time as discussed by J. L. Summerdijk and A. Bril, in "Visible Luminescence . . . Under I.R. Excitation", Internation Conference on Luminescence, Leningrad, August 1972, p. 86. In the case of deep traps, trapped electrons can be released at a later time by photons having an energy similar to the depth of the trap. Thermal discharging is neglible in the case of deep traps. Under these circumstances, it appears that light has been "stored" for later use and emission of visible light can be activated by infrared. In the case of shallow traps, spontaneous emission will occur at room temperature because the thermal agitation is sufficient to excite electrons out of the traps. These materials are now called electron trapping optical materials.
The fundamentals of electron trapping material are the following: A host crystal is a wide bandgap semiconductor (II-VI) compound, normally without any special value. These crystals, however, can be doped heavily with impurities to produce new energy levels and bands. Impurities from the lanthanide (rare earth) series are some of the elements that can be accomodated in the lattice to form a "communication" band and a trapping level. The new communication band provides an energy band in which the untrapped electrons can interact. The trapping level at yet lower energies represents non-communicating sites.
Materials that display latent luminescent activity often include one or more types of sites where electrons may be trapped in an energized state. Upon application of suitable wavelengths of energizing radiation, such as visible light or x-rays, such sites become filled with electrons. The energizing radiation results in the electrons being raised to an energized state via the communication band from which transitions, such as absorption and recombination, may take place. Upon removal of the energizing radiation, the electrons may be trapped at an energy level higher than their original ground state or may drop back to their original ground state. The number of electrons that become trapped is very much dependent upon the composition of the photoluminescent material and the dopants used therein.
If the trapping level is sufficiently below the level of the communication band, the electrons in them will be isolated from each other, will rmain trapped for a long period of time, and will be unaffected by normal ambient temperatures. Indeed, if the depth of the trap is sufficient, the electrons will remain trapped almost indefinitely unless they are activated by specific light energies, or thermal energy much higher than room temperature.
The electrons will remain trapped until light or other radiation is applied to provide sufficient energy to again raise them to the level of the communication band, where a transition may take place in the form of recombination, allowing the electrons to escape from the trap and release photons of visible light. The material must be such that room temperature thermal energy is insufficient to allow any significant portion of trapped electrons to escape from their traps. As used herein, "optical energy" shall include visible light, infrared light, and ultraviolet light unless otherwise noted, "photoluminescent material" is a material that exhibits the above characteristics.
Although various photoluminescent materials have heretofore been known, the properties have often been less than desirable. For example, photoluminescent materials have been used for locating infrared beams by outputting visible light upon placement of the material within an infrared beam, but such previous photoluminescent materials are not sensitive enough to detect relatively low levels of infrared radiation. The visible light output by such materials is often at a very low level such that detection of the visible light is difficult. Further, such materials commonly have electron traps with insufficient depth and/or a relatively low density of electron traps such that it is difficult to maintain the electrons trapped for extended periods of time. The ratio of the energy of light input to energy of light output in such materials is often quite high. That is, a relatively large amount of energy must be put into the material to provide a given output optical energy. The development of photoluminescent materials that avoid or minimize the disadvantages discussed above would open up numerous other applications for such materials.